Various gas barrier films and packaging materials produced from them have already been known. The best gas barrier material is aluminum foil. However, aluminum foil easily suffers pinholes when used singly, and can serve only for special purposes. Consequently, aluminum film is used only as an interlayer in laminate films in most cases. Aluminum-containing laminate films have very good gas barrier properties. However, such laminate films are opaque, and the contents of a bag made of such a laminate film cannot be seen through it. There are other disadvantages including the difficulty in determining whether the bag has been heat-sealed completely and unsuitability as packaging material for foods to be cooked in a microwave oven.
Thermoplastic films such as polyester film and polyamide film are high in strength, transparency, and moldability, and have been used widely as packaging material. These thermoplastic resin films, however, are high in permeability to gases such as oxygen and vapor. If used as packaging material for general, boiled, and retort-pouch foods, therefore, the contents can undergo deterioration and degradation as they are stored for a long time.
As a solution to this problem, films of polyolefin, nylon, polyethylene terephthalate (hereinafter abbreviated as PET), and the like, coated with an emulsion of vinylidene chloride (hereinafter abbreviated as PVDC) and the like have been used conventionally as packaging material required to have gas barrier properties such as for foods. Films with a PVDC layer formed by coating have a high barrier function against oxygen not only at low humidity but also at high humidity, and also have good barrier properties against vapor. If PVDC-coated films are incinerated for disposal, however, chlorine gas can be generated from the chlorine contained in the PVDC, and in addition, dioxins can also be produced. Thus, PVDC-coated films can have serious adverse influence on environment and human health, and development of alternative materials has been strongly called for.
The most widely known chlorine-free gas barrier materials include polyvinyl alcohol (hereinafter abbreviated as PVA) film and films coated with PVA or an ethylene-vinyl alcohol copolymer (hereinafter abbreviated as EVOH). PVA and EVOH show very good barrier properties against oxygen gas in a dry environment. These materials, however, have problems as follows: (1) their barrier properties depend largely on humidity and the barrier properties deteriorate seriously under high-humidity conditions, (2) they do not have a barrier function against vapor, (3) they can be easily dissolved in hot water, and (4) they suffer large deterioration in gas barrier properties caused by water absorbed during the boiling and retorting process.
There is a proposal of vapor-deposited films that have a vapor-deposited film of an inorganic oxide such as, for instance, aluminum oxide and silicon oxide formed by physical vapor deposition such as vacuum deposition on one of the surfaces of a thermoplastic film such as polyester film. These gas barrier films with a thin deposited inorganic oxide layer have the advantages of being transparent to make the contents visible and suitable for cooking in a microwave oven. The gas barrier layer is generally stiff, however, in the case of films having a deposited inorganic oxide layer as gas barrier layer. Consequently, they have the disadvantage of suffering a serious deterioration in gas barrier properties as a result of cracks and pinholes produced in the gas barrier layer when bent.
To solve this problem, there are generally known techniques that improve gas barrier properties and flexibility by providing a thermoplastic resin film with a deposited inorganic oxide layer, followed by further coating the deposited layer with a polymer to form a gas barrier layer (for instance, Patent documents 1 to 3).    Patent document 1: Japanese Unexamined Patent Publication (Kokai) No. 2004-35833    Patent document 2: Japanese Unexamined Patent Publication (Kokai) No. 2002-307600    Patent document 3: Japanese Unexamined Patent Publication (Kokai) No. HEI-9-327882
The technique described in Patent document 1 is designed to provide a polyester base film with a gas barrier layer by crosslinking the polymer through ester bonds in order to improve the barrier properties at high humidity. To allow the esterification to proceed sufficiently to improve the gas barrier properties of the film, the film should be heated to cause the reaction at a high temperature. Undesirably, this leads to a decrease in productivity.
Patent document 2 describes a technique that provides a base film of polyester, polypropylene, and the like with a thin inorganic oxide film, and forms, on top of it, a transparent coated layer composed of a thermosetting epoxy resin and an organic curing agent. When undergoing hot-water sterilization such as boiling and retorting, however, a film of this layered structure suffers a large deterioration in the contact between the inorganic oxide layer and the transparent coated layer. Specifically, when used as packaging material for foods, this film can suffer practical problems such as delamination (interlayer peeling) of the surface film when subjected to hot water sterilization such as boiling and retorting.
In addition, the technique described in Patent document 3 forms a primer layer formed with a primer agent between a base film and a deposited inorganic oxide layer. Films of such a layered structure can develop a strong contact between a deposited inorganic oxide layer and a gas barrier layer when subjected to hot water sterilization such as boiling and retorting. However, they require an increased number of production steps, leading to the problem of increased production costs.
The present invention was made in view of the above problems, and aims to provide a gas barrier film that does not cause environmental contamination with halogens, have good barrier properties against gases such as oxygen and vapor, and resist hot water sterilization such as boiling and retorting.